Effectiveness of screening for breast cancer in women under 50 years at entry: the Kotka pilot project in Finland.

It is still uncertain whether mammography is beneficial to women under 50 years of age. We report a follow-up of 4 1-year birth cohorts in 13 Finnish municipalities subjected to service screening. The women in these cohorts were born in even calendar years and they were 40-47 years old at the time of first screening, which was repeated every other year. At the beginning, 4,163 women were invited and 86% attended. Women were screened with mammography, physical breast examination and information on detection and early diagnosis of breast cancer by breast self-examination (BSE) was given. Incident cases of breast cancer and breast-cancer deaths were recorded. The expected numbers of incident breast-cancer cases and deaths from breast cancer were derived from control women who were residents of the same municipalities but who were born in adjacent calendar years (odd years) to those screened. In the screened cohorts 13 cancers were screen-detected, 32 cancers were interval cases and 8 cases were diagnosed among the non-attenders, while in the controls 85 cancers were detected during a 9-year follow-up from 1982 to 1990. One death from breast cancer occurred in the screened cohorts among cases diagnosed between 1982 and 1990, against 10 expected breast-cancer deaths during a 10-year follow-up from 1982 to 1991 of women initially free of breast cancer. Breast-cancer mortality in the screened cohorts was significantly lower than in the controls (Rate Ratio 0.11), but it is not clear how much of the reduction in mortality could be accounted for by the mammography itself and how much to other activities included in the screening programme (especially training in BSE) or to chance variation. It is unlikely that a programme sensitivity of 25% (13/53) would have resulted in 89% (100-11) effectiveness or reduction in the risk of death from breast cancer.